Consumption of trichlorfon-poisoned fish by women in a small Hungarian village has been associated with trisomy resulting from an error of meiosis II in oogenesis. We therefore examined mouse oocytes exposed for 3 h during fertilization to 50 microg/ml trichlorfon. Spindle morphology was not visibly altered by the pesticide. Chromosomes segregated normally at anaphase II with no induction of aneuploidy. However, formation of a spindle was disturbed in many oocytes resuming meiosis I in the presence of trichlorfon. In spite of the spindle aberrations and the failure of bivalents to align properly at the equator, oocytes did not become meiotically arrested but progressed to metaphase II. At this stage, spindles were highly abnormal, and chromosomes were often totally unaligned, unattached or dispersed on the elongated and disorganized spindle. By causing spindle aberrations and influencing chromosome congression, trichlorfon appears, therefore, to predispose mammalian oocytes to random chromosome segregation, especially when they undergo a first division and develop to metaphase II during exposure. This is the first case in which environmentally induced human trisomy can be correlated with spindle aberrations induced by chemical exposure. Our observations suggest that oocytes may not possess a checkpoint sensing displacement of chromosomes from the equator at meiosis I and may therefore be prone to nondisjunction.